A retro-reflector is a device that sends light or other radiation back towards the direction it came from regardless of the angle of incidence, unlike a mirror, which does that only if the mirror is exactly perpendicular to the light beam. This effect can be commonly obtained in two ways: with a series of three perpendicular mirrors (a corner reflector), a sphere of material with refractive index different from the surrounding media and/or other properly designed geometries.
Retro-reflection is used on road surfaces, road signs, vehicles and clothing (large parts of the surface of special safety clothing, less on regular coats). When lights illuminate a retro-reflective surface, the reflected light is directed generally towards the source, and not highly wasted by going in all directions as with diffuse reflection. Retro-reflectors can approach efficiencies greater than 60%. Corner reflectors are better at sending the light back to the source over long distances, while spheres are better at sending the light to a receiver somewhat off-axis from the source.
In 1942, Engineering News—Record wrote, “Paint surfaced with reflective beads has been found superior to any other type painted pavement marking Five hundred miles of this type have been laid in Philadelphia and found to be very satisfactory. Although glass-beaded paint costs more, experience shows that it wears four to five times as long.” When reflectorized paint was first introduced, the greater durability of the paint line made the reflectorized paint more cost effective. Today, reflectors are used on center and edge lines for greater safety.
FIGS. 1 and 2 illustrate the difference between using pavement markings with and without reflective beads. FIG. 1 shows prior art pavement markings with uncoated retro-reflective beads at night when illuminated with visible light from automobile headlights. FIG. 2 shows prior art pavement markings without reflective beads at night when illuminated with visible light from automobile headlights. By comparing FIGS. 1 with 2, it can be seen that the use of even uncoated reflective beads makes a dramatic difference. The two center lines here reflected yellow light, as the uncoated beads were embedded in yellow paint. Up to 80% of the signal can be reflected back to source with a 60° from normal acceptance angle.
Unbeaded paint lines will reflect light randomly in all directions. When round reflective beads are added, light is reflected and directed toward the source of the light.
Hence, a retro-reflector is a device that sends light or other radiation back where it came from regardless of the angle of incidence, unlike a mirror, which does that only if the mirror is exactly perpendicular to the light beam. This effect can be commonly obtained in two ways: with a set of three perpendicular mirrors (a corner reflector) and with a transparent sphere of material with refractive index. A retro-reflector may consist of many very small versions of these structures incorporated in a thin sheet or in paint. In the case of paint containing glass beads, the paint glues the beads to the surface where retro-reflection is required, and the structure protrude, about twice the thickness of the paint. A third, much less common way of producing a retro-reflector is to use the nonlinear optical phenomenon of phase conjugation. This technique is used in advanced optical systems such as high-power lasers and optical transmission lines.
When light strikes a non-corner reflector retro-reflector it is refracted and reflected. Refraction is the bending of the light. Refraction is observed when a pencil is dropped into a half filled glass of water; the pencil appears bent. Reflective retro-reflector's ability to bend light is measured by its index of refraction, which is a ratio of the sine of the angle of incidence to that of the refraction. The retro-reflectivity of glass beads is better explained by examining the path of light as it enters a single retro-reflector in the paint (FIG. 3). There are actually millions of tiny retro-reflector in each delivery element that perform this principle.
As the laser beam enters the retro-reflector, it is bent or refracted. This beam then shines on the back surface of the retro-reflector, which is on top of the paint, thermoplastic, etc. The paint works a lot like a mirror. If the paint were not present, most of the light would continue through the bead and bounce in several directions. This is one reason why the retro-reflectors are often placed in a polypropylene paint up to one half of the retro-reflector. The light is bent (refracted) by the curved surface of the retro-reflector to a point below where the retro-reflector is sunk into the paint. Thus, when light is reflected off the paint at the back of the reflectors a large portion of that light is reflected through the retro-reflector and refracted back toward the light transmitter.
The amount of refraction of light is characteristic of the glass itself and is known as the index or refraction (n) of the glass, bead, particle, or phosphor. The refractive index of the retro-reflector is dependent upon the chemical and physical make-up of the glass material. Various types of retro-reflectors have different indices of refraction and cause different amounts of light to be retro-reflection or returned to the receiver/laser transmitter.
Water has an index of refraction of 1.33, while the typical bead made with soda glass has a refractive index of 1.50. Beads used in the pavement marking industry are available in refractive indexes of 1.50, 1.65 and 1.90. The durability of the glass with an index of refraction of 1.9 is not as good as the soda glass retro-reflector. Retro-reflectors with a refractive index of 1.90 are generally called, “airport beads,” and are used to mark runways at airports.
Retro reflectivity is dependent upon the immersion level on the bead, particle, or phosphor. Optimum level of retro-reflector beads is 50-60%, assuring optimum imaging. Increasing immersion beyond 60% significantly decreases the amount of light that can be directed back to the receiver/laser. Typically this coating is obtained when the bead is set into a paint or polymer.
The problem of cargo security is global. Some reported numbers include
Airport/Port Security                9,618,337 departed flights in a recent year        63,800 Cargo ship port calls in 2007        
Cargo Containers                $12.5 Trillion of cargo is moved each year in ˜200 million containers        Globally, 2% are inspected        Nearly $50 billion a year is lost to high-value cargo theft        
Border Control                41,694,587 pedestrians crossed US borders in a recent year        6,626,007 loaded truck containers        194,525,561 passengers in personal vehiclesIllegal immigrants are estimated at 12 million, almost 1 in every 20 workers with∘26 tunnels in 2009, 16 in 2008 with a 60% increase in tunneling.        
Product Counterfeiting
The overall cost of counterfeiting in the world was about 5-7% of world trade (some estimates put this at $176-250 billion)